228 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxviii. 



18fil. Otililfiiufc BiLLiNcis, (Jeology of Vernioiit, II, p. 94i), figs. 3r)0-;!5L*. 

 1S()1. Ov7///'.s/;/a Bn;i,iN(is, Paleozoic Fo^^nil^, I, p. 10, figs. II, 12. 



1862. (>/7/(/,s'//wf Hillings, Paleozoic Fossils, I, p. ^'M, lig. 115. 



1863. OrthlK Hall, 16t,h Kept. N. Y. State Cab. Nat. Hist., p. 134, pi. vi, figs. 23-27. 

 1863. Orthisina Billings, Geology of Canada, p. 284, fig. 289. 



1867. Orthisina Hall, Trans. Albany Institute, V, p. 113. 



1868. Orthis Hartt, Dawson's Acadian Geology, Second Edition, p. 644, 



fig. 233. 



1882. Orthis Whitfield, Geology of Wisconsin, IV, p. 170, pi. i, figs. 4, 5. 



1883. Orthis {Orthisina f) Hall, Report of State Geologist, N. Y., pi. xxxvii, 



figs. 16-19. 



1884. Orthinina Whitfield, Bull. Am. Mus. Nat. Hist., I, p. 144, pi. xiv, fig. 6. 

 1884. Orthis Walcott, Bull. No. 10, U. S. Geol. Survey, p. 17, pi. i, figs. 1, la-d. 



?1884. Orthis Walcott, Mongr. U. S. Geol. Survey, VIII, p. 22, pi. ix, figs. 8, 8a. 

 1886. Orthisina Walcott, Bull. No. 30, U. S. Geol. Survey, pp. 120-122, pi. vii, 



figs. 5-7. 

 1886. Orthis Matthew, Trans. Roy. Soc. of Canada, Sec. IV, p. 43, pi. v, figs. 



20a-c; (?) p. 42, pi. v, figs. 18a-c. 

 1892. BiUingsella Hall and Clarke, Pal. New York, VIII, Pt. 1, p. 230. 



1896. Orthis {BiUingsella) Sardeson, Bull. Minnesota Acad. Nat. Sci., IV, 



p. 96. 



1897. Eillingsella Schuchert, Bull. U. S. Geol. Survey, No. 87, p. 158. 



Diagnosis. — Shell subqimdrate or subsemicircular in outline. Con- 

 tour unequally biconvex or plano-convex. Shell punctate. (?) Surface 

 striate or plicate. Ventral valve with the greatest convexity; cardinal 

 area moderately high, slightly inclined outward; delthyrium covered 

 by a convex plate which in the type species B. color adoerisis has a 

 minute perforation near the apex; teeth well developed, with dental 

 plates extending to the bottom of the umbonal cavity and forming the 

 sides of a small area opposite the delthyrium, where the pedicle mus- 

 cle was probably attached. In the dorsal valve the cardinal area is 

 strongly inclined, but less than 90° to the plane of the valve. The 

 delthyrium is partially covered by a convex chilidium, or it may be 

 altogether absent. Cardinal process single. 



Type. — OrtJds coloradoensis Shumard. 



OhseTvations. — Messrs. Hall and Clarke founded the genus on Orthis 

 pepina Hall, which is a synonym of Ortliis coloradoenslx Shumard. 

 The material from the limestones of Texas in the collections of the 

 Survey show the form of the teeth in the ventral valve, also the delti- 

 dium and the minute perforation near its apex. Messrs. Hall and 

 Clarke state in their diagnosis that "in rare instances [it] may be 

 minutely perforated at the apex," but do not name the species in 

 which the perforation occurs or where the specimens showing it are 

 to be found. All the species known to me are biconvex or plano- 

 convex; none are concavo-convex as defined by Messrs. Hall and 

 Clarke. 



The genus is essentially orthoid, but it differs in the presence of the 

 arched deltidium and its general aspect. 



